Longitudinal study on the occurrence in pigs of colistin resistant E. coli carrying mcr-1 following the cessation of use of colistin.

AimsIn 2015, colistin-resistant Escherichia coli and Salmonella with the mcr-1 gene were isolated from a pig farm in Great Britain. Pigs were subsequently monitored over a similar to 20-month period for the occurrence of mcr-1-mediated colistin resistance and the risk of mcr-1 E.coli entering the food chain was assessed. Methods and ResultsPig faeces and slurry were cultured for colistin-resistant E.coli and Salmonella, tested for the mcr-1 gene by PCR and selected isolates were further analysed. Seventy-eight per cent of faecal samples (n=275) from pigs yielded mcr-1 E.coli after selective culture, but in positive samples only 02-13% of the total E.coli carried mcr-1. Twenty months after the initial sampling, faecal samples (n=59) were negative for E.coli carrying mcr-1. ConclusionsThe risk to public health from porcine E.coli carrying mcr-1 was assessed as very low. Twenty months after cessation of colistin use, E.coli carrying mcr-1 was not detected in pig faeces on a farm where it was previously present. Significance and Impact of the StudyThe results suggest that cessation of colistin use may help over time to reduce or possibly eliminate mcr-1 E.coli on pig farms where it occurs.